fix: frontmatter & api hmr bug when enable mfsu

[PeachScript]

🤔 这个变动的性质是？/ What is the nature of this change?

• 新特性提交 / New feature
• bug 修复 / Fix bug
• 样式优化 / Style optimization
• 代码风格优化 / Code style optimization
• 性能优化 / Performance optimization
• 构建优化 / Build optimization
• 网站、文档、Demo 改进 / Website, documentation, demo improvements
• 重构代码或样式 / Refactor code or style
• 测试相关 / Test related
• 其他 / Other

🔗 相关 Issue / Related Issue

Close #795
Close #841
Close #917

💡 需求背景和解决方案 / Background or solution

优化临时文件访问和 theme context 初始化方案，确保 MFSU 开启后 FrontMatter 及 API 能正常热更新，主要改动点：

1. 临时生成 .umi/dumi/layout.jsx 用于配合 theme/layout 做 context 初始化，theme/layout 不直接依赖 @@/dumi/config
2. 把 @@/dumi/demos 及 @@/dumi/apis 都挪进 theme context 中，内层组件统一走 context 访问数据

改动后可确保 dumi 主题包及 preset-dumi 的 theme 模块不会依赖 .umi/dumi 下的临时文件，可以正常、安全地被 MFSU 预编译。

📝 更新日志 / Changelog

Language	Changelog
🇺🇸 English	Fix FrontMatter & API HMR bug when enable MFSU
🇨🇳 Chinese	修复 MFSU 开启后 FrontMatter 及 API 无法热更新的问题

[github-actions]

🎊 PR Preview has been successfully built and deployed to https://dumi-preview-pr-959.surge.sh

[github-actions]

🎊 PR Mobile Preview has been successfully built and deployed to https://dumi-mobile-preview-pr-959.surge.sh

[codecov]

Codecov Report

Merging #959 (0bf0cea) into master (60a781a) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #959      +/-   ##
==========================================
+ Coverage   94.70%   94.74%   +0.04%     
==========================================
  Files         117      117              
  Lines        2853     2857       +4     
  Branches      971      972       +1     
==========================================
+ Hits         2702     2707       +5     
+ Misses        147      146       -1     
  Partials        4        4              

Impacted Files	Coverage Δ
...ges/preset-dumi/src/plugins/features/demo/index.ts	100.00% <ø> (ø)
packages/preset-dumi/src/routes/getRouteConfig.ts	89.18% <ø> (ø)
packages/preset-dumi/src/transformer/remark/api.ts	94.64% <ø> (+1.66%)	⬆️
packages/theme-mobile/src/layouts/index.tsx	100.00% <ø> (ø)
packages/preset-dumi/src/loader/index.ts	100.00% <100.00%> (ø)
...ackages/preset-dumi/src/plugins/features/routes.ts	100.00% <100.00%> (ø)
packages/preset-dumi/src/plugins/features/theme.ts	100.00% <100.00%> (ø)
packages/preset-dumi/src/theme/hooks/useApiData.ts	100.00% <100.00%> (ø)
packages/preset-dumi/src/theme/layout.tsx	87.30% <100.00%> (+0.41%)	⬆️

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